Production of fluorocarbons by catalytic pyrolysis of fluoroform

ABSTRACT

Fluorocarbons (other than fluoroform) are obtained by pyrolysing fluoroform at 500 DEG -1200 DEG C. in the presence of a catalyst comprising an activated carbon, a fluoride of a metal of Group Ia or IIa of the Periodic Table, or aluminium fluoride, this last including catalysts obtained by the fluorination of alumina and still containing oxygen. The activated carbon may be used as such, or impregnated with Group IIa metal chlorides, such as MgCl2, SrCl2 or BaCl2. Specified metal fluorides are NaF, KF, LiF, RbF, CsF, MgF2, CaF2, SrF2 and BaF2. Aluminium fluoride catalysts may be obtained by fluorinating alumina with HF, CF2Cl2, CF2ClCF2Cl, CF2ClCFCl2 or CHF3; or AlF3 may be used, as such or supported on a non-siliceous carrier such as corundum or fused beryllia or thoria.  The products obtained are saturated perfluorocarbons and fluorohydrocarbons and olefinic products. In examples the main products are CF4, C2F6, C2HF5, C2F4, C3F6, C3F8, C4F8 and C4F10.ALSO:Catalysts suitable for the pyrolysis of fluoroform (see Division C2) comprise AlF3 supported on a non-siliceous carrier such as corundum or fused beryllia or thoria.

United States Patent f 3,222,406 PRODUCTION OF FLUOROCARBONS BY CATA- LYTIC PYROLYSIS 0F FLUGROFORM Murray Hauptschein, Glenside, and Arnold H. Fainberg,

Elkins Park, Pa., assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvama No Drawing. Filed Dec. 28, 1961, Ser. No. 162,975 4 Claims. (Cl. 260-653) This application is a continuation-in-part of the copending application Serial No. 159,172, filed December 13, 1961, now abandoned, of Murray Hauptschein and Arnold H. Fainberg.

This invention relates to the production of fluorocarbons by the pyrolysis of fluoroform in the presence of metal fluoride catalysts.

The pyrolysis of fluoroform to produce fluorocarbons is described in US. Patent 3,009,966, of Murray Hauptschein and Arnold H. Fainberg. As set out in detail in that patent, it was found that the uncatalyzed pyrolysis of fluoroform produces perfluoroolefins, particularly tetrafluoroethylene and hexafluoropropene, as the major product, with only small quantities of saturated fluorocarbons.

In accordance with the present invention it has now been found that when the pyrolysis of fluoroform is carried out in the presence of certain metal fluorides, the production of saturated fluorocarbons, and particularly saturated perfluorocarbons in the C to C range, is greatly increased at the expense of the unsaturated products. A salient feature of the invention is its ability to produce good yields of fluorocarbons in the C to C., range, i.e., C 1 C F and C F These materials, prized for their unusual chemical inertness and thermal stability, are useful as low temperature refrigerants, heat exchange fluids and dielectric media.

The metal fluoride catalysts useful in the process of the invention include the fluorides of the metals of groups IA and IIA of the periodic table of elements. In group IA, sodium fluoride and potassium fluoride are preferred although lithium fluoride, rubidium fluoride and cesium fluoride can also be used. The preferred fluorides of group IIA include magnesium fluoride, calcium fluoride, strontium fluoride and barium fluoride. Sodium fluoride is particularly preferred in that it exhibits particular selectivity for the production of C to C saturated perfluorocarbons. Preferably, the sodium fluoride catalyst consists of sodium fluoride prepared by the removal of HP from sodium bifiuoride, NaHF by heating at a temperature of 300 C. or higher.

The catalyst can be used in any desired form, e.g., in the form ofia fixed bed of pellets, e.g., A5 to /2" in size, or as a fluidized bed of fine particles in accordance with well known fluidized bed techniques. When employed as a fixed bed of pellets, the reactor may conveniently consist of tubes having a diameter of, e.g., /2" to 6" packed with pellets of the desired catalyst. The catalyst bed can be heated by any desired means such as by an electric furnace surrounding the catalyst-packed tube. The catalyst tubes should be constructed of materials resistant to attack by the reactants or reaction products at the operating temperatures. Preferred materials of his type include for example nickel, platinum, stainless steel, Inconel, Monel metal or the like.

During the course of the reaction, free carbon is gradually deposited on the catalyst. This causes a gradual decrease in the activity of the catalyst and after a sufliciently long period, will cause plugging of the catalyst bed. For these reasons, it is desirable to periodically remove this by-product carbon by passing an oxygen containing gas such as air at a controlled temperature, e.g.,

3,222,406 Patented Dec. 7, 1965 500 to 800 C. through the bed to oxidize carbon to CO and CO and thus remove it from the catalyst.

The catalytic pyrolysis reaction of the invention is carried out at temperatures of from about 500 to 1100 C., but in no event higher than the melting point of the particular metal fluoride employed. Optimum temperatures of operation will generally lie in the range of 650 to 950 C. where the best combination of relatively high conversions and good yields of saturated perfluorocarbons in the C to C range will generally be obtained.

The contact time of the reactants with the catalyst may vary over a wide range depending upon the temperature employed. As the temperature increases, shorter contact times are used. Thus, contact times as long as 10 minutes at the lower temperatures to as short as 0.01 second at the upper temperature limit may be employed. In the preferred temperature range of from 650 to 950 C., contact times of from 0.5 to 60 seconds will generally be used. As used herein, contact time is defined as follows:

Contact time (seconds) volume occupied by catalyst bed Reaction pressure is not critical and may be atmospheric, sub-atmospheric or super-atmospheric. While atmospheric pressure operation will generally be found most convenient, sub-atmospheric pressures, ranging as low as about 25 mm. Hg as a practical limit may be found useful in some cases. Super-atmospheric pressures may range, e.g., up to about 10 atmospheres.

The composition of the pyrolysis products will vary somewhat depending upon the reaction conditions, particularly the temperature and the particular catalyst chosen. In general, higher temperatures somewhat favor higher ratios of unsaturated to saturated fluorocarbons, and accordingly, optimum yields of saturated fluorocarbons are obtained in the intermediate range of temperatures of 650 C. to 950 C. The olefinic products are principally hexafluoropropene and C olefins, mainly perfluoroisobutylene. If desired, the olefinic product may be recycled with fresh fluoroform feed to increase the overall yields of saturated fluorocarbons.

In addition to saturated perfluorocarbons ranging from CF, to C.,F there is often produced appreciable amounts of perfluorocarbon monohydrides, such as C HF and CaHFq. These are formed by the addition of hydrogen fluoride (split off during the pyrolysis) to perfluoroolefins also formed in situ during the pyrolysis. In this regard the metal fluoride catalyst also acts as a catalyst for the addition of hydrogen fluoride to the perfluoro-olefins particularly tetrafluoroethylene.

While the invention does not depend upon any particular reaction mechanism, it is probable that the reaction of the invention involves the following stoichiometry for the various products formed:

The presence of hydrogen fluoride in the reaction product and the deposition of free carbon on the catalyst in the course of the reaction both lend support to the above reaction course.

The following examples illustrate the invention.

Equation Equation Equation Equation Equation Equation Equation Equation Equation Examples 1 to 6 A center 15" section of a nickel tube having an inside diameter of 'As" is packed with 152.5 grams (170 milliliters bulk volume) of sodium fluoride tablets, A X As" fluoroform to various products is determined as in Examples 1 to 6. The results of five runs at temperatures from 600 to 800 C. are summarized in Table II.

in size. The sodium fluoride catalyst is prepared by heat- 5 Example 12 8 tablets of soillm blflllel'ide NaHEz to a temperature Using the same equipment and technique described in P excess of to dnve Q The catallfst e Examples 1 to 11, fluoroform is passed over a bed of is heated by an nsulated electric furnace concentric with catalyst pellets of barium fluoride at a temperature of ahe tutb; and 24 l1n llenggl ip e t te e: l i i 10 900 C. and at a space rate of 100 volumes of fluoroform a 1 512 2 3335; li g g e on er 0 per volume of catalyst per hour. A mixture of saturated Fluoroform is passed through catalyst bed at and unsaturated fluorocarbons mostly in the C to C various temperatures and at a space velocity of 60 volf l g g g 3: F swivel-5102s ts umes of fluoroform (calculated at S.T.P., i.e., 0 C. and 0 as 00 a e orfagomg E C lmen 760 Hg) per volume of catalyst bed per hour (Com 15 of the lnvention are for purposes of illustration only and tact time of about 15 to seconds). The reactor that the iQvention is not limitgd thereto eflluent is passed through a hydrogen fluoride scrubber We 01mm: consisting of a tube packed with sodium fluoride in pellet A methed of Prepaflng flueroeafbens Whleh form held at 100 C. where hydrogen fluoride is removed, prlses py y g fiuoroform 1n the presence f a metal and then collected in refrigerated receivers. P od t 20 fluoride catalyst selected from the class consisting of the analyses are made by gas-liquid chromatography and fluorides of the metals of groups IA and IIA of the peinfrared spectra. From the total product analyses, the riodic table of elements, at a temperature below the meltpercent conversion of fluoroform to each product is deing point of said metal fluoride in the range of from 500 termined on the basis of the stoichiometry shown in c to 1100 C Equations 1 to 9 above. The results obtained in a series 2 A method f preparing fl o bo which of 51X runs temperatures Varymg from 6000 to prises pyrolyzing fluoroform in the presence of a metal "i g i y be Seen that as th 1 fluoride catalyst selected from the class consisting of the e fluorides of the metals of groups IA and IIA of the neperatur? mcreasias at constant .Space veloclty the total riodic table of elements at a temperature below the meltconverslon also increases. It Wlll be noted that saturated t f m tal fluorid catal St in th 1 f fluorocarbons, principally C 1 C 1 and C F were the mg P o Sal 0 e e y 6 range tom principal products. The yield of unsaturated material 650 to g d 1 (C F and C F somewhat increases as the temperature A P O accor Wlt c mm 2 m whlch Said increases. In Example 6, run at the same temperature as catalyst 13 Potasslum Example 5, a somewhat lower conversion was obtained A methed for PreParlHg flu01Oea1'b0I1S Whleh e after a longer period 'of running apparently due to the p e Pyrolylmg fluefeferm 1n the Presence of SOdluHl deposition of further amounts of free carbon on the fluorlde at a temperature In the range of from C- to catalyst.

TABLE I Percent conversion of fluorol'orm toxamp e 6 Total Total Total product saturates uns.tu- CF4 02F oaFs 04 30 C2HF5 Ca v CaFa C4Fa Others Excluding small amounts of unidentified materials. b Includes n-+is0-C4Fm. 9 Includes OFr=C(CF and cisand trans-CF3CF=CFCFa. Unidentified, mostly higher boiling material.

TABLE II f T Percent conversion of fluoroform to emp., E 1 C.

Xamp 6 Total Total Total CF4 C2176 OaFa CzHFn CaHF7 C4HF9 C Fg C4F3 product saturates unsaturates Examples 7 to 11 References Cited by the Examiner The same equipment is used as in Examples 1 to 6 but UNITED STATES PATENTS the bed of sodium fluoride pellets is replaced by 260 2,551,573 5/ 1951 Downing et al. 260-653 grams of 4;" tablets of potassium fluoride. Fluoroform 70 3,009,966 11/ 1961 Hauptschein et a1. 260653 is passed through the catalyst bed at a space velocity of 3,016,405 1/ 1962 Lovejoy 260-653 60 volumes of fluoroform (calculated at S.T.P.) per volume of catalyst per hour (contact time of about 15 to 20 seconds) at various temperatures ranging from 600 to 800 C. Product analyses and percent conversion of LEON ZITVER, Primary Examiner.

ALPHONSO D. SULLIVAN, DANIEL D. HQRWITZ,

Examiners. 

1. A METHOD OF PREPARING FLUOROCARBONS WHICH COMPRISES PYROLYZING FLUOROFORM IN THE PRESENCE OF A METAL FLUORIDE CATALYST SELECTED FROM THE CLASS CONSISTING OF THE FLUORIDES OF THE METALS OF GROUPS IA AND IIA OF THE PERIODIC TABLE OF ELEMENTS, AT A TEMPERATURE BELOW THE MELTING POINT OF SAID METAL FLUORIDE IN THE RANGE OF FROM 500* C. TO 1100*C. 